Failback to primary datacentre when using EMC VNX 5500 and SRM

We are planning a full DR test whereby we fail our VM's over to the secondary datacenter using SRM 5.0 (this will be a full recovery, not a ring fenced test)

We are unsure on how the VNX works when it comes to failing back.

One of our engineers believes this will require a Full re sync from the VNX at the secondary back to the Primary before we can failback, this would require replicating 12TB of data.

I would have thought if the LUN's at what was the primary site are still in tact then it would only replicate the changes that have been made since the failover.

Can anyome who have ever done this with a VNX confirm, this will have a massive bearing on our DR test as we are planning on failing over, running from DR site for 4 hours then failing back, if we have to wait until a full replication from secondary to primary before we can fail back then it will take days so wont be able to do it in our weekend time window